Period prediction is off

So, is anyone else's period prediction on here off? Like mine is off by a good week. I have an app that I've had for awhile but it looked really early teenagery so, that's why I got glow. And when I got glow and another app, I went through both and put in all of my past periods going back to January 2014. I've watched it for a couple of months now, app 1 & app 2 are in sync ( and right ) about when my period comes.. But glow is completely way off. I like how glow is set up and the community part of it.. But it's not predicting right and I was just wondering if other people were having this problem.